count = 0
n = input()
if 999 < int(n) < 10000:
    for left in range(4):
        for right in range(left + 1, 4):
            num = int(n[left:right+1])
            c = 0
            for d in range(2, num // 2):
                if num % d == 0:
                    c = 1
                if c != 1 and num > count:
                    count = num
print(count)